It is well known that aromatic compounds can be alkylated in the aromatic nucleus by reaction with various alkylating agents in the presence of a protonic acid or a Lewis acid as catalyst. Such is known as the Friedel-Crafts reaction. A very wide range of acidic catalysts, of which aluminum chloride may be the best known example, have been used to prepare a wide variety of alkyl-substituted benzene derivatives.
Such reactions are of great importance and of wide applicability. They can, however, suffer from at least three principal limitations. First, it can be rather difficult to prevent di-alkylation of the benzene nucleus, since the first product formed, the monoalkylated product, tends to be more reactive than the starting material. Thus, it is often necessary to conduct the reaction in such a way that only a relatively small proportion of the aromatic starting material reacts, and then to separate off the product or products formed, and recycle the starting material. On an industrial scale, such procedures can be very costly.
Second, when using certain substituted benzenes as starting materials, alkyl substitution can lead to two or more positional isomers. In some cases, one isomer is formed in large excess over the other possible isomer; in many cases, however, the reaction is relatively unspecific, and mixtures of some or all of the possible isomers are formed.
Third, as is stated clearly in a well known textbook:
" . . . aromatic rings containing the --NH.sub.2, --NHR, or --NR.sub.2 group do not undergo Friedel-Crafts alkylation, partly because the strongly basic nitrogen ties up the Lewis acid needed for ionization of the alkyl halide: ##STR1## Tying up of the acidic catalyst by the basic nitrogen is not the only factor that prevents alkylation, since when excess catalyst is used, reaction does not occur". R. T. Morrison and R. N. Boyd, Organic Chemistry, third edition, (Boston) 1974.sup.5, page 382, Chapter 12.8, "Limitations of Friedel-Crafts Alkylation".
Only one exception to this general rule has been discovered, in 1955, by Stroh, et al. (German Auslegeschrift No. 1,051,271; U.S. Pat. No. 3,275,690), who found that it is possible to alkylate aromatic amines using liquefied lower olefins in the presence of, e.g. aluminum chloride, at high temperatures and pressures, e.g. 300.degree. C. and 250 atm. Under these severe conditions the aromatic nucleus is alkylated, selectively, but only at the position(s) ortho to the amino group. Only if both ortho-positions are already occupied by substituents, and then not always, para-alkylation may take place. The highest molar ratio of aluminum halide to the aniline used is 0.028.
Accordingly, the art has not provided a process suitable for the large-scale selective alkylation of aromatic amines (anilines) to 4-alkylanilines or N-alkyl congeners.